grep

Search file(s) for specific text.

Syntax
grep [options] PATTERN [FILE...]
grep [options] [-e PATTERN | -f FILE] [FILE...]
A simple example:
$ grep "Needle in a Haystack" /etc/*
Options
-A NUM
--after-context=NUM
Print NUM lines of trailing context after matching lines.
Places a line containing -- between contiguous groups of matches.
-a
--text
Process a binary file as if it were text; this is equivalent
to the --binary-files=text option.
-B NUM
--before-context=NUM
Print NUM lines of leading context before matching lines.
Places a line containing -- between contiguous groups of matches.
-b
--byte-offset
Print the byte offset within the input file before each line of output.
--binary-files=TYPE
If the first few bytes of a file indicate that the file contains binary data, assume
that the file is of type TYPE. By default, TYPE is binary, and grep normally outputs
either a one-line message saying that a binary file matches, or no message if there
is no match. If TYPE is without-match, grep assumes that a binary file does not match;
this is equivalent to the -I option. If TYPE is text, grep processes a binary file as
if it were text; this is equivalent to the -a option.
Warning: grep --binary-files=text might output binary garbage, which can have nasty
side effects if the output is a terminal and if the terminal driver interprets some
of it as commands.
--colour[=WHEN]
--color[=WHEN]
Surround the matching string with the marker from the GREP_COLOR environment variable.
WHEN can be 'never', 'always', or 'auto' e.g. --color=always By default the matched text will be colored red.
If grep is made to match several strings, all of the matches will be colored, one exception
is the regex ^ (match beginning of every line), the beginning of a line has no length so will
not be coloued.
So to return all lines and colour only matches: egrep --color=always '^|string1|string2'
-C NUM
--context=NUM
Print NUM lines of output context.
Places a line containing -- between contiguous groups of matches.
-c
--count
Suppress normal output; instead print a count of matching lines for each input file.
With the -v, --invert-match option (see below), count non-matching lines.
-D ACTION
--devices=ACTION
If an input file is a device, FIFO or socket, use ACTION to process it.
By default, ACTION is read, which means that devices are read just as if they were
ordinary files. If ACTION is skip, devices are silently skipped.
-d ACTION
--directories=ACTION
If an input file is a directory, use ACTION to process it.
By default, ACTION is read, which means that directories are read just as if they
were ordinary files. If ACTION is skip, directories are silently skipped.
If ACTION is recurse, grep reads all files under each directory, recursively; this
is equivalent to the -r option.
-E
--extended-regexp
Interpret PATTERN as an extended regular expression.
-e PATTERN
--regexp=PATTERN
Use PATTERN as the pattern; useful to protect patterns beginning with -.
-F
--fixed-strings
Interpret PATTERN as a list of fixed strings, separated by newlines,
any of which is to be matched.
-f FILE
--file=FILE
Obtain patterns from FILE, one per line. The empty file contains zero
patterns, and therefore matches nothing.
-G
--basic-regexp
Interpret PATTERN as a basic regular expression This is the default.
-H
--with-filename
Print the filename for each match.
-h
--no-filename
Suppress the prefixing of filenames on output when multiple files are searched.
--help
Output a brief help message.
-I
Process a binary file as if it did not contain matching data; this is
equivalent to the --binary-files=without-match option.
-i
--ignore-case
Ignore case distinctions in both the PATTERN and the input files.
-L
--files-without-match
Suppress normal output; instead print the name of each input file from which
no output would normally have been printed. The scanning will stop on the first match.
-l
--files-with-matches
Suppress normal output; instead print the name of each input file from which
output would normally have been printed. The scanning will stop on the first match.
-m NUM
--max-count=NUM
Stop reading a file after NUM matching lines.
If the input is standard input from a regular file, and NUM matching lines are
output, grep ensures that the standard input is positioned to just after the last
matching line before exiting, regardless of the presence of trailing context lines.
This enables a calling process to resume a search. When grep stops after NUM matching
lines, it outputs any trailing context lines.
When the -c or --count option is also used, grep does not output a count greater
than NUM. When the -v or --invert-match option is also used, grep stops after
outputting NUM non-matching lines.
--mmap
If possible, use the mmap(2) system call to read input, instead of the default
read(2) system call. In some situations, --mmap yields better performance.
However, --mmap can cause undefined behavior (including core dumps) if an input
file shrinks while grep is operating, or if an I/O error occurs.
-n
--line-number
Prefix each line of output with the line number within its input file.
-o
--only-matching
Show only the part of a matching line that matches PATTERN.
--label=LABEL
Display input actually coming from standard input as input coming from file LABEL.
This is especially useful for tools like zgrep, e.g. gzip -cd foo.gz |grep --label=foo something
--line-buffered
Use line buffering, it can be a performance penality.
-P
--perl-regexp
Interpret PATTERN as a Perl regular expression.
-q
--quiet, --silent
Quiet; do not write anything to standard output.
Exit immediately with zero status if any match is found, even if an error
was detected. Also see the -s or --no-messages option.
-R
-r
--recursive
Read all files under each directory, recursively; this is equivalent to the '-d recurse' option.
Not all grep implementations support -r and among those that do, the behaviour with symlinks may differ.
--include=PATTERN
Recurse in directories only searching file matching PATTERN.
--exclude=PATTERN
Recurse in directories skip file matching PATTERN.
-s
--no-messages
Suppress error messages about nonexistent or unreadable files.
Portability note: unlike GNU grep, traditional grep did not conform to POSIX.2 ,
because traditional grep lacked a -q option and its -s option behaved like
GNU grep's -q option. Shell scripts intended to be portable to traditional grep
should avoid both -q and -s and should redirect output to /dev/null instead.
-U, --binary
Treat the file(s) as binary.
By default, under MS-DOS and MS-Windows, grep guesses the file type by looking at
the contents of the first 32KB read from the file. If grep decides the file is a
text file, it strips the CR characters from the original file contents
(to make regular expressions with ^ and $ work correctly).
Specifying -U overrules this guesswork, causing all files to be read and passed
to the matching mechanism verbatim; if the file is a text file with CR/LF pairs
at the end of each line, this will cause some regular expressions to fail.
This option has no effect on platforms other than MS-DOS and MS-Windows.
-u
--unix-byte-offsets
Report Unix-style byte offsets.
This switch causes grep to report byte offsets as if the file were a Unix-style text
file, i.e. with CR characters stripped off. This will produce results identical to
running grep on a Unix machine.
This option has no effect unless -b option is also used; it has no effect on
platforms other than MS-DOS and MS-Windows.
-V
--version
Print the version number of grep to standard error.
This version number should be included in all bug reports (see below).
-v
--invert-match
Invert the sense of matching, to select non-matching lines.
-w
--word-regexp
Select only those lines containing matches that form whole words.
The test is that the matching substring must either be at the beginning of
the line, or preceded by a non-word constituent character. Similarly, it
must be either at the end of the line or followed by a non-word constituent
character. Word-constituent characters are letters, digits, and the underscore.
-x
--line-regexp
Select only those matches that exactly match the whole line.
-y
Obsolete synonym for -i.
-Z
--null
Output a zero byte (the ASCII NUL character) instead of the character that
normally follows a file name. For example, grep -lZ outputs a zero byte after
each file name instead of the usual newline.
This option makes the output unambiguous, even in the presence of file names
containing unusual characters like newlines.
This option can be used with commands like find -print0, perl -0, sort -z, and
xargs -0 to process arbitrary file names, even those that contain newline characters.

Environment variables

Grep's behavior can be affected by setting the following environment variables

GREP_OPTIONS - default options
GREP_COLOR - The marker for highlighting
LC_ALL, LC_COLLATE, LANG - These variables specify the LC_COLLATE locale,
which determines the collating sequence used
to interpret range expressions like [a-z].
LC_ALL, LC_COLLATE, LANG These variables specify the LC_MESSAGES locale, which determines the
language used for messages. The default C locale uses American English messages.
LC_ALL, LC_CTYPE, LANG - specify the LC_CTYPE locale, which determines the type of
characters, e.g., which characters are whitespace.
LC_ALL, LC_MESSAGES, LANG
These variables specify the LC_MESSAGES locale, which determines the
language that grep uses for messages. The default C locale uses American English messages.
POSIXLY_CORRECT - Posix behaviour
_N_GNU_nonoption_argv_flags_
If the ith character of this environment variable's value is 1, do not consider
the ith operand of grep to be an option, ( N is grep's numeric process ID)
see `info grep' for more

Grep stands for: Global Regular Expression Print.

Exit Status

grep exits with one of the following values:
0 One or more lines were selected.
1 No lines were selected.
>1 An error occurred - syntax error in pattern, inaccessible input files, or other system error.

The `-v' option can be used to effectively invert the exit status.

Examples

Search the file example.txt, including binary data (-a) for the string 'hunting the snark':

$ sudo grep -a 'hunting the snark' example.txt

Search the whole partition (/sda1), including binary data(-a) for the string 'hunting the snark' return all the lines starting 25 Before the text found and 50 lines After the matching text found.
This can be a way to discover fragments of deleted files but is very slow:

$ grep -a -B 25 -A 50 'hunting the snark' /dev/sda1 > results.txt

Search the file wordlist.txt for any lines that don't include at least one vowel:

$ grep -v [aeiou] wordlist.txt

Remove lines from invoices.txt if they appear in paid.txt:

$ grep -F -x -v -f paid.txt invoices.txt >paidinvoices.txt

"I understand that change is frightening for people, especially if there's nothing to go to. It's best to stay where you are.
I understand that" ~ Princess Diana